Gold nanoparticles possess a variety of size-dependent optical phenomena that can be harnessed for applications such as the detection of analytes in solution, the construction of microscale optical devices, and biomedical assays or treatments. Effective application of those properties is realized by controlling the nanoparticle surface chemistry (functionality). The principal attribute of gold nanoparticles harnessed in these applications is the surface plasmon resonance (SPR) phenomenon that manifests itself as an intense absorption at 520 nm. The intensity of this absorption and energy at which it occurs are determined by the size of the gold nanoparticle. Because gold nanoparticles possess such important size-related effects, the ability to control the size of the gold nanoparticle is an important factor of an effective nanoparticle synthesis. However, for many applications, precise size control is useless if the functionality of the particle cannot also be controlled. It is the surface chemistry of the particle that enables many of the applications detailed above.
A number of chemical strategies have been developed for synthesizing functionalized gold nanoparticles of various sizes. Yet even the most commonly used techniques (e.g., the Brust synthesis and ligand-exchange techniques) still suffer from limitations that make it a challenge to synthesize gold nanoparticles of a particular size with precisely controlled surface chemistry. In the Brust synthesis, gold nanoparticles are synthesized in the presence of functionalized thiols, producing thiolate monolayer-protected gold nanoparticles. However, because gold nanoparticle growth is quickly passivated by the formation of thiolate bonds, gold nanoparticles with core diameters greater than 4 nm cannot effectively be made in this way.
In contrast, ligand-exchange techniques require a stepwise synthesis approach that involves producing a gold nanoparticle stabilized by an exchangeable ligand (citrate, phosphines, or amines) that is replaced with a functionalized thiol in a subsequent step to yield the monolayer-protected particle. Such methods, however, cannot be used to make large AuNPs in an efficient and reproducible manner because incomplete ligand exchange becomes a problem and the reaction is sensitive to changes in ionic strength and interactions between incoming and outgoing functionalities. The differences in reaction efficiency may be related to the curvature of the particle, the reactivity of the particle surface, and other size-related changes in the surface chemistry of the particle.
Accordingly, traditional approaches to making gold nanoparticles face limits to and challenges in the ability to make functionalized gold nanoparticles having desired core sizes, and there is a significant long-felt need for the development of methods that produce specifically functionalized gold nanoparticles with tuneable core sizes.